Protection of piezoelectric crystals



May 4, 1948. D. s. YOUNG PROTECTION OF PIEZO ELECTRIC CRYSTALS Filed Nov. 28, 1945 FIG.2

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DEAN S. YOUNG Patented May 4, 1.948

PROTECTION OF PIEZOELECTRIC YSTALS v Dean 8. Young, Bethesda, Md. Application November 28, 1945, Serial No. 631,462 9 Claims. (01. 171-327) (Granted under the act of March amended April 30, 1928; 370 0.

This invention relates to the use of a fusible protective link in connection with a piezoelectric crystal. More particularly this invention relates to novel features of construction of a crystal holder designed to increase the protection of a party guaranteeing continued satisfactory operation of such crystals.

The increased number of radio transmitters occupying a given frequency spectrum and the increased precision of frequency maintenance required for military, commercial and amateur pur poses has greatly increased the number of piezoelectric crystals in active use. Because of the exacting manufacturing process such crystals are necessarily expensive and are ordinarily guaranteed to give satisfactory performance by the manufacturer or jobber.

Frequently crystals are returned to the manufacturer for replacement on a no-charge basis. The manufacturer has no way of knowing whether the crystals failed in normal use or because of excessive current flow caused by misuse. Because of the large number of crystals which are returned for replacement because of misuse, the manufacturer must charge considerably more for crystals in order to defray the expense of replacement.

It is an object of my invention to provide a crystal protective means designed to open the crystal circuit upon pas-sage of a current of predetermined magnitude.

It is another object of my invention to include overcurrent protective means in the crystal holder to limit crystal current to a non-damaging value.

It is a further object of this invention to provide a crystal holder including fusible protective means arranged in such a way that the party guaranteeing the successful performance of the crystal is protected to the maximum extent.

It is a still further object of my invention to provide a crystal holder utilizing a commercial type easily-procurable fuse having low inductance and distributed capacitance.

It is still another object of my invention to provide a crystal holder wherein the crystal and protective means are assembled in such a manner as to enable the inductance and distributed capacitance of the assembly to be a minimum.

It is still another object of my invention to disclose a mounting for a cartridge fuse in a crystal holder in which good electrical contact is obtained without danger of breakage of the fuse body. a

It is a further object of my invention to pro- 3, 1883, as G. 757) vide a crystal holder in which an electric lamp is used as an indicating and protective means.

In the drawings: Figure 1 is a vertical section showing one em-- 5 bodiment of my invention;

Figure 2 is a vertical section showing another embodiment of my invention in which fuse replacement may be effected without disturbing the crystal.

Figure 3 is a vertical section showing use of an electric lamp as a protective and indicating means.

In Figure 1 is shown a simple embodiment of my invention in which the numeral 10 indicates the base or body portion of the holder. Projectingfrom the base are two terminals, l2 and it. Terminal I2 is connected to a crystal electrode it by some resilient means of good conducting properties, for example, the spring l8. The crystal 20 bears against the upper plate 22 which serves as the upper electrode. The upper plate is attached to the base by means of machine screws 24. Adjacent the crystal receptacle I! is a fuse receptacle 28 containing a fuse 28. Contact with the fuse ferrules occurs at points 30 and 32, at which points may be provided conical upraised contacts or means for applying resilient pressure.

The machine screws 24 may include a hole for the passage of a sealing wire 34 by the manufacturer. The sealing wire may be sealed by a lead pellet or by any one of a number of ways well known to those skilled in the art. Such seal makes it impossible for one other than the manufacturer to remove either the fuse or the crystal without the manufacturer being aware of such removal. Under normal circumstances the breakage of the seal may be cause to void the crystal guarantee. 1

By use of the disclosed structure the inclusion of the fuse in the circuit will not affect the properties of the crystal. A fuse marketed under the name "Littelfuse is readily available and possesses the advantages of low inductance and low distributed capacitance. Normally such fuses are mounted in clips applying lateral pressure to the fuse ferrules. An increase of the pressure to obtain firmer contact runs the danger of breakage of the tubular glass body. It will be noted that in the structure which I disclose terminal pressure is applied axially. Thus an increase of terminal pressure to obtainbetter contact causes the upraised terminals to dig into the solder at the ends of the fuse or to cause the ferrule ends to be deformed inwardly to a small extent. Such inwarddeformation is not accompanied by lateral pressure on the glass wall; thus good contact is assured even though the length of the fuses used may vary. Since the fuse terminal is in contact with the same electrode 22 which contacts the crystal and since the other ferrule of the fuse is in close proximity to the terminal [4, the overall distributed capacitance and inductance of the fuse circuit is kept to a minimum.

The external appearance of the crystal holder which I disclose may be made similar if not identical to holders not including fusible protective means. Thus, if desired, the manufacturer need not state at time of purchase that the holdercontains a fuse. At such time as the crystal is returned for replacement under the guarantee, the manufacturer may inspect the fuse. If the fuse is burnt out and the crystal has suffered no damage it is merely necessary for the inexpensive fuse to be replaced and thesame crystal and holder returned to the former user or sold for re-use.

Since the fuse is normally chosen so that the circuit is interrupted just short of the damage point of the crystal, burning out of the fuse is conclusive evidence that the crystal has been misused. Such misuse may occur by operating the fuse at higher than rated current over a relatively long period of time or by operating the fuse at a greately excessive current for a short period of time. The conventional commercial type of fuse has been found to protect the crystal under both low overload and high overload conditions since it has an inverse current-time burnout characteristic.

In Figure 21 have shown a modified form of crystal holder in which a setscrew 38 engages the cover plate 22 to retain the fuse 28 in place. The setscrew is preferably provided with a sharpened point or upraised portion 38 to engage the ends of the ferrule and thus to provide firmer contact. This structure has the advantage that the fuse may be removed without affecting the crystal 28. This prevents the disturbing of the crystal with respect to its electrodes which may affect its oscillating properties and also prevents the entry of dirt particles, grease and the like.

The structure of Figure 2 is particularly well adapted -for fuse replacement by the user of the crystal. The rating of the fuse may be stamped or otherwise imprinted upon the body of the fuse holder. This structure has the additional advantage that the contact pressure applied to the fuse may be adjusted to an optimum value to compensate for fuses of varying lengths.

In Figure 3 an, embodiment is shown in which an electric lamp 40 is used both as an indicating means to give an indication of crystal current and as a current limiter. In this modification the body portion 42 is provided with a well or receptacle 44 in which is seated a crystal electrode 48; Placed upon the crystal 48 is another electrode 50 which is kept in resilient engagement with the crystal, for example by means of a conducting spring 52. Placed on top of the base or body member 42 is a metallic plate 54 in which a tapped hole 58 is provided for threadedly engaging the lamp 40. The central conducting tip of the lamp may engage the conducting spring 52 in any desired manner. The simplest means of obtaining such contact has been illustrated in Figure 3.

Superimposed upon the metallic plate 54 is a cap 58, which may be made of transparent plastic containing a depression 68 to receive the bulb portion of the lamp 48. To keep the assembly together machine screws 82 are provided for threaded engagement with the base 42. Such screws may be sealed in at the factory by covering the heads with sealing compound 84 and distinctively moulding the surface. In order to provide contact between plate 54 and terminal I4 a metallic insert 88 is provided which may be raised slightly above the frame of the body portion 42 to provide a bearing surface. If desired the plastic cap may be provided with depending edges 88 thus making it impossible for any internal conducting elements to come in contact with the user.

The lamp 40 is chosen to correspond with the current conducting capabilities of the crystal. Preferably the lamp should have a damage curve which lies parallel to and slightly below a damage curve of the crystal. This embodiment has the advantage that crystal currents in excess of the rated values but not sufficient to cause damage to the crystal may be signaled to the operator by the lighting of the lamp. A further advantage in using a lamp, particularly one with a tungsten filament, is that the resistance increases greatly from the cold to the heated condition to provide more or less automatic protective effect. As in the embodiment shown in Figures 1 and 2, the holder may be sealed at the factory in order to protect the manufacturer under the guarantee.

While I have described my invention with particular reference to the use of commercial type cartridge fuses because of their ready availability, it is important to note that fuses may be constructed for this application which have burnout characteristics parallel the damage characteristic curve of the type of crystal used more closely than commercial crystals. Such fuses may be made having a link of zinc, zinc alloy or many other types of link material of suitable characteristics known to those skilled in the art. Also if desired the fuse link may be made of tungsten or other substance having a large increase in resistance with temperature. The increase in resistance has a current limiting protective effect which will automatically protect the crystal from short duration current peaks which may. for example, occur during tuning of the apparatus.

It will be seen from the above that I have provided a crystal holder well suited to protecting a party guaranteeing the operation of a piezoelectric crystal. It will also be seen that I have provided a mounting which is inexpensive to manufacture, which uses a readily procurable type of protective element and which insures that the inductance, capacitance and resistance of the circuit is substantially no greater than if the crystal alone were used.

While the invention is susceptible of various modifications and alternative constructions, I have shown in the drawings and have described in detail only the preferred embodiments. It is to be understood, however, that I do not intend to limit the invention by such disclosure, for I aim to cover all modifications and alternative constructions falling within the spirit and scope of the invention as defined in the appended claims.

The invention described herein may be. manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor.

What is claimed is:

1. A crystal holder comprising a body having a crystal receptacle, said body also containing a receptacle for a cartridge fuse, a single metallic cover member extending over both said receptacles, acrystal, and a fuse, said cover member providing an electrode for said crystal within said crystal receptacle and an abutting contact for said fuse within said fuse receptacle.

2. A crystal holder comprising a body portion containing receptacles for a crystal and a fuse, a metallic cover member to provide electrical contact between said crystal and said fuse, means to keep said cover plate and said body member in contact, and a tampering indicator seal at tached to said means.

3. A crystal holder comprising crystal-receiving electrodes, fusable protective means associated with said crystal-receiving electrodes, a cover, and means to maintain said cover in place, said cover being so constructed and arranged as to conceal the presence of said protective means within said holder.

4. A crystal holder comprising a body member having a crystal well and a fuse well, a cartridge fuse and a crystal mounted in said wells respectively, and a metallic cover arranged to apply contact pressure to said crystal and to said fuse, said fuse well being arranged perpendicular to said cover whereby contact pressure is applied axially to said fuse.

5. A crystal holder comprising a body member having a fuse Welland a crystal well, a metallic cover plate, and conductive retaining means projecting through said cover plate and positioned above said fuse well, said retaining means serving both to maintain said fuse within said body mem her and to apply electrical contact pressure to said fuse.

6. A fuse holder comprising a body member having a crystal Welland a fuse well, external contacts, contacts at the base of said fuse well and said crystal well respectively, said contacts being respectively connected to said external contacts, a metallic cover plate covering said crystal well, a plug engaging said cover plate and positioned over said fuse well, said plug serving to retain said fuse in position, and tampering indicator sealing means associated with said plug.

7. A fuse holder comprising a body member having a crystal well, upper and lower crystal electrodes in said well, a lamp receptacle positioned above said well, an electric lamp mounted in said receptacle, electrical conduction means between said lamp and said upper electrode, a cover having a depression to receive said lamp, and fastening means engaging said cover and said body member to maintain them in assembled relation.

8. A crystal holder comprising a body portion containing a crystal well, an upper and a lower crystal electrode located in said well substantially parallel to the mouth thereof, a metal plate containing a lamp receptacle, a lamp mounted in said receptacle, said lamp containing a contact tip, said contact tip being in electrical and mechanical engagement with said upper electrode, cover means, and sealing means associated with said cover means, said cover means preventing removal of said lamp or access to said crystal except after defacing of said sealing means.

9. In a crystal holder a crystal, protective means to limit the current flowing through said crystal, means necessarily damaged upon the disassembly of said holder, said holder being so constructed and arranged that the external appearance conceals the presence of said protective means.

DEAN S. YOUNG.

REFERENCES CITED UNITED STATES PATENTS Name Date Eberhard Aug. 4, 1931 Number 

